Effect of laser hardening and post hardening shot peening on residual stress evolution in X5CrNiCuNb16-4 steel for steam turbine blade applications

Abstract

Laser-hardening process produces higher surface hardness and provides deeper hardness profile into the material. This increment in hardness reduces water-droplet erosion on steam turbine blades. In the present study, a steam turbine blade profile was subjected to laser-hardening process primarily on the suction side of the profile. This hardening process transformed tempered martensite structure of the unaffected material to a fine martensite structure at the processed zone with an intermediate transition zone being present in between them. The fine martensite was responsible for spike in hardness profile at the surface. When the laser-hardened blade was subjected to operational stresses, a number of parallel cracks formed on the blade profile in the direction perpendicular to the blade axis. Detailed investigations in the form of residual stress mapping revealed that a narrow interfacial region beside laser-hardened zone over the profile of the processed blade material, indicated presence of tensile residual stresses. This region was responsible for the formation of parallel cracks over the blade profile. A shot peening process was introduced post laser-hardening which resulted in the elimination of the tensile residual stress, thus, mitigating crack formation over the blade profiles upon application of operational stresses.